The invention relates to phenol-free phosphites, which can be used to stabilize organic polymers, especially polyvinyl chloride (“PVC”).
Liquid organic phosphites have been used for many years alone and in combination with mixed metal stabilizers for the stabilization of vinyl halide polymers, especially PVC. The Encyclopedia of PVC, Volume 1, L. Nass, Ed., Marcel Dekker Inc., New York, 1977. The phosphite esters employed may be trialkyl, triaryl, mixed alkyl/aryl, and even polymeric.
The problem of imparting polyvinyl chloride with sufficient heat processing stability at temperatures at which the polymer becomes sufficiently fluid or softened to permit shaping is of course of long standing, and has been satisfactorily resolved by the addition to the polymer of various combinations of known heat stabilizers. At processing temperatures, the PVC resin can degrade, liberating hydrogen chloride, discolor, become brittle, and stick to the processing equipment. These problems are overcome by combining with the polymer before heat processing or during heat processing, one or more of the well established and conventional heat stabilizers, such as, for example, alkyl tin mercaptides or barium/cadmium or barium/zinc or calcium/zinc salt mixed metal stabilizers, aryl, alkyl and mixed aryl/alkyl phosphites, or combinations of the above.
These stabilizers, in preventing the deterioration of the polymers during processing at high temperatures, also permit manufacture of products with increased intrinsic quality because of the enhancement of their resistance to thermal and light degradation during use. In addition, because of the ability of these products to withstand more rigorous conditions, their versatility is increased and new areas of application are thereby opened. Without going into details or theory, it has been found that mixed alkyl/aryl phosphites such as diphenylisodecyl phosphite and phosphites based on pentaerythritol give the best overall performance in combination with mixed metal stabilizer systems for the stabilization of PVC.
In recent years there has been much concern with exposure to volatiles from the processing of PVC resin, and the exposure to volatiles from articles shaped from stabilized PVC resin exposed to elevated use temperatures. The volatilization of one or more components, or of the decomposition products therefrom, cause the condensation of these volatile components as “fog” on surfaces adjacent to the PVC articles. It has been found that one of the volatiles from the processing of PVC containing certain stabilizers is phenol. The phenol comes from the phosphite used in combination with the mixed metal stabilizer. There is a great need to eliminate or at least minimize the phenol content of phosphite stabilizers and still have a stabilizer which gives good color and processing stability and is relatively inexpensive.
One important objection to the contamination of PVC resins with phenol is based on the use of vinyl chloride polymers in food applications, e.g. in the manufacture of food containers. The use of phenol-free stabilizers prevents the transfer of objectionable odors or materials to food. Another consideration is the fact that for other applications, phenol has been identified as the source of the cause of premature discoloration, and is a water-soluble component.
A preferred phosphite for use with mixed metal stabilizers is diphenyl isodecyl phosphite, but this stabilizer contains about 50% of total phenol. Other phosphite stabilizers based on dialkyl pentaerythritol diphosphites have been known for some time as effective stabilizers for vinyl polymers. Despite wide usage as stabilizers for vinyl chloride polymers, polyolefins, polyurethanes, styrene polymers, and ABS, this type of phosphite has not been entirely satisfactory. The reason for this is the fact that, because of the method of preparation, namely by transesterification from triphenyl phosphite, the dialkyl pentaerythritol diphosphite is contaminated with phenol. Also, pentaerythritol phosphites are prone to hydrolysis that liberate solid pentaerythritol. In addition, it is advantageous to use mixed metal/phosphite stabilizer combinations as a single liquid component added to the PVC resin during processing. The dialkyl pentaerythritol diphosphites mentioned above are not easily combined with the liquid mixed metal stabilizers, and on standing, a mixture of the liquid mixed metal stabilizer and the dialkyl or diaryl pentaerythritol diphosphite separate into a lighter liquid layer and a more solid layer of heavy sludge. The dialkyl or diaryl pentaerythritol diphosphite also has a tendency to separate from the PVC matrix on compounding, causing a phenomenon known as plate-out. This lack of package stability and formation of plate-out greatly reduces the usefulness of this type of phosphite.
It is also known that trialkyl phosphites, while providing good early stability and color as well as good initial ultraviolet stability, suffer from poor long-term stability. Triaryl phosphites by contrast, provide good long-term stability, but suffer from early color and ultraviolet stability. Bis-phenol A diphosphites are recognized to provide excellent performance, but at a cost which is higher than commercially viable for many applications.
Pentaerythritol type phosphites and vinyl resins stabilized with such phosphites are disclosed in U.S. Pat. No. 3,281,381 by I. Hechenbleikner and F. C. Lanoue. These pentaerythritol phosphites are prepared by the transesterification of triphenyl phosphite with pentaerythritol to give, depending on the molar ratio of the triphenyl phosphite and pentaerythritol, a variety of possible structures. Tetra-phosphites are made by using four (4) moles of the tri(aromatic phosphite), such as triphenyl phosphite for each mole of the pentaerythritol. “Spiro” products, which are diphosphites, are made from the reaction of two moles of the triaryl phosphite with one mole of pentaerythritol. Mixed cyclic and non-cyclic esters are made by the reaction of three moles of the starting phosphite with each mole of pentaerythritol. Although the phenol formed in the transesterification reaction used to produce these materials from triphenylphosphite is removed by distillation during the preparation, the products still contain small quantities of free phenol, and phenol bound as a phosphite ester may be liberated during compounding or mixing.
Hechenbleikner, in U.S. Pat. No. 3,205,250 suggests the use of dialkylpentaerythritol diphosphites as stabilizers for polyvinyl chloride. Such dialkyl pentaerythritol diphosphites are prepared according to U.S. Pat. No. 4,206,103 by the reaction of two moles of an alkyl alcohol with a diphenyl- or dichloropentaerythritol diphosphite, made by the reaction of two moles of triphenylphosphite or phosphorous trichloride with one mole of pentaerythritol. When diphenyl pentaerythritol diphosphite is the reactant, the spiro isomer comprises about half the combined total of spiro and caged isomers in the product. When dichloropentaerythritol diphosphite is substituted for the diphenyl pentaerythritol diphosphite, the product which results is the relatively pure spiro isomer, which is generally a solid.
The preparation of dialkylpentaerythritol diphosphites which are not contaminated by the presence of phenol is disclosed in U.S. Pat. No. 4,290,976. The process disclosed utilizes the dichloropentaerythritol diphosphite made from phosphorous trichloride and pentaerythritol as a starting material since it does not contain a phenyl group and there is no possibility of phenol being formed as a contaminant. The products of the process described are characterized by higher set points and as a result do not form stable one-phase mixtures with liquid mixed metal stabilizers.
U.S. Pat. No. 3,047,608 describes the preparation of trialkyl phosphites and dialkyl pentaerythritol diphosphites by transesterification from triphenyl phosphite using a dialkyl or diphenyl phosphite as catalyst. The completeness of the transesterification and the removal of the byproduct phenol is controlled by the addition of an excess of the higher aliphatic alcohol, and removal of that excess along with the residual phenol by slow co-distillation under vacuum. The use of the diphenyl phosphites as catalysts, however, is not efficient, and the product from reaction of pentaerythritol with two moles of triphenyl phosphite and four moles of the higher aliphatic alcohol is mostly in the spiro form, and is incompatible with liquid mixed metal stabilizers.
U.S. Pat. No. 6,362,260 describes liquid organic phosphites of low volatility, based on pentaerythritol, alkyl alcohols and alkyl phenols, which are essentially phenol-free, but which require a pentaerythritol core structure.
Accordingly, there remains a need for essentially phenol free phosphites (as used in this application to mean less than or equal to 0.5%) which have good compatibility, performance, low volatility, package stability with mixed metal stabilizers and good performance, eliminating the drawbacks typically associated with current state-of-the-art phenol-based phopshites such as didecyl phenyl phosphite or diphenyl decyl phosphite.